Power source for a dispersion compensation fiber optic system

ABSTRACT

This invention generally relates to an optical filter for a fiber optic communication system. An optical filter may be used, following a directly modulated laser source, and converts a partially frequency modulated signal into a substantially amplitude modulated signal. The optical filter may compensate for the dispersion in the fiber optic transmission medium and may also lock the wavelength of the laser source.

[0001] This application claims priority to two U.S. provisionalapplications: (1) U.S. Application No. 60/395,161, filed Jul. 7, 2002;and (2) U.S. Application No. 60/401,419, filed Aug. 6, 2002, which areboth hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention generally relates to a power source for a fiberoptic system that converts a partially frequency modulated signal into asubstantially modulated signal and compensates for dispersion in atransmission fiber.

[0004] 2. General Background and State of the Art

[0005] Fiber optic communication systems use a variety of transmittersto convert electrical digital bits of information into optical signalsthat are sent through optical fibers. On the other end of the opticalfiber is a receiver that converts the optical signal to an electricalsignal. The transmitters modulate the signals to form bits of 1s and 0sso that information or data may be carried through the optical fiber.There are a variety of transmitters that modulate the signal indifferent ways. For example, there are directly modulated transmittersand indirectly modulated transmitters. The directly modulatedtransmitters offer a compact system having large response to modulationand are integrateable. The directly modulated transmitters are alsogenerally less expensive than the externally modulated transmitters,which require an intensity modulator, usually LiNbO3, following thelaser. One of the drawbacks of a directly modulated transmitter,however, is that its output is highly chirped. Chirp is the rapid changein optical frequency or phase that accompanies an intensity modulatedsignal. Chirped pulses become distorted after propagation through tensof km of dispersive optical fiber, increasing system power penalties tounacceptable levels. This has limited the use of directly modulatedlaser, transmitters to applications with limited distances of tens of kmat 2.5 Gb/s as described by P. J. Corvini and T. L. Koch, Journal ofLightwave Technology vol. LT-5, no. 11, 1591 (1987). For higher bit rateapplications, the use of directly modulated transmitters may be limitedto even shorter distances.

[0006] An alternative to directly modulating the laser source is using alaser source that produces a partially frequency modulated signal and anoptical discriminator as discussed in UK Patent GB2107147A by R. E.Epworth. In this technique, the laser is initially biased to a currentlevel high above threshold. A partial amplitude modulation of the biascurrent is applied so that the average power output remains high. Thepartial amplitude modulation also leads to a partial but significantmodulation in the frequency of the laser output, synchronous with thepower amplitude changes. This partially frequency modulated output maythen be applied to a filter, such as a Fabry Perot filter, which istuned to allow light only at certain frequencies to pass through. Thisway, a partially frequency modulated signal is converted into asubstantially amplitude modulated signal. That is, frequency modulationis converted into amplitude modulation. This conversion increases theextinction ratio of the input signal and further reduces the chirp.

[0007] Since Epworth, a number of variations from his technique havebeen applied to increase the extinction ratio from the signal output ofthe laser. For example, N. Henmi describes a very similar system in U.S.Pat. No. 4,805,235, also using a free-space interferometer. Huber U.S.Pat. No. 5,416,629, Mahgerefteh U.S. Pat. No. 6,104,851, and BrennerU.S. Pat. No. 6,115,403 use a fiber Bragg grating discriminator insimilar configurations. In the more recent work, it has also beenrecognized that a frequency-modulated transmitter with a frequencydiscriminator produces an output with lower chirp, which reduces thepulse distortion upon propagation through a communication fiber. Chirpis a time dependent frequency variation of an optical signal andgenerally increases the optical bandwidth of a signal beyond theFourier-transform limit. Chirp can either improve or degrade the opticalpulse shape after propagation through a dispersive fiber, depending onthe sign and exact nature of the chirp. In the conventional directlymodulated laser transmitter, chirp causes severe pulse distortion uponpropagation through the optical fiber. This is because the speed oflight in the dispersive medium is frequency dependent, frequencyvariations of pulses may undergo different time delays, and thus thepulse may be distorted. If the propagation distance through the mediumis long as in the case of optical fibers, the pulse may be dispersed intime and its width broadened, which has an undesirable effect.

[0008] In the above systems, the discriminator is operated to increasethe extinction ratio of the input signal or to remove some component ofthe signal in favor of the other. As such, only the amplitude variationof the discriminator has been utilized. In addition, these systems havemainly dealt with lower bit rate applications. At low bit rates, thespectrum of a modulated laser biased above its threshold includes twocarriers, each carrying the digital signal used to modulate the laser.The wavelengths of the two peaks are separated by 10 GHz to 20 GHzdepending on the laser and the bias. Hence, a variety of opticaldiscriminators, Fabry-Perot, Mach-Zehnder, etc. may be used to resolvethe two peaks, generally discarding the 0s bits and keeping the 1s bits,thereby increasing the extinction ratio at the output.

[0009] A Fabry-Perot filter is formed by two partially reflecting mirrorsurfaces, which are separated by a small gap on the order of a fewmicrometers. The cavity is either an air gap or a solid material formedby deposition or cut and polish method. The transmission of aFabry-Perot filter consists of periodic peaks in optical frequencyseparated by the so-called free-spectral range (FSR), which is inverselyproportional to the thickness of the gap. The steepness of the peaks isdetermined by the reflectivities of the two mirrors. However, thesteeper the transmission edges, the narrower the pass-band of thefilter. As such, Fabry-Perot filter may provide the steeper transmissionedges or slope, but it does not provide the broad enough bandwidth forhigh bit rate applications such as 10 Gb/s.

[0010] At higher bit rates, the spectrum of the frequency modulatedsignal becomes more complicated and the choice of discriminators thatmay be used is limited. At high bit rates around 10 Gb/s, theinformation bandwidth becomes comparable to the frequency excursion ofthe laser, which is typically between 10 GHz to 15 GHz. In addition, thetransient chirp that arises at the transitions between 1s and 0sbroadens to complicate the spectrum further. In order to separate the 1and 0 bits with the extinction ratio of 10 dB, the slope of thediscriminator should be greater than 1 dB/GHz, while passing 10 Gb/sinformation. Under these performance criteria, a Fabry-Perot filter maynot work because the bandwidth and slope characteristics of Fabry-Perotfilters are such that the steeper the transmission edges, the narrowerthe pass-bandwidth of the filter. As illustrated in FIGS. 1A and 1B, aFabry-Perot discriminator with 1 dB/GHz slope may only have about 3 GHzbandwidth. Such limited bandwidth can severely distort a 10 Gb/s signalsuch that the FM modulated transmitter with a Fabry-Perot filter may notwork at this bit rate. Others have tried fiber Bragg gratings for highbit rate applications, but these are sensitive to temperature andrequire separate package with temperature stabilization. Therefore,there still is a need for a discriminator that can operate with a FMmodulated source at high bit rates without being sensitive totemperature changes.

SUMMARY OF THE INVENTION

[0011] This invention provides an optical discriminator capable ofoperating with a frequency modulated (FM) source at high bit rates andhaving dispersion that is opposite sign of the dispersion in thetransmission fiber to neutralize at least some portion of the dispersionin the fiber. With the discriminator providing dispersion that isopposite sign of the dispersion in the fiber, signal degradation due todispersion in the fiber is minimized. This invention also provides amodulated laser source and a discriminator system that compensates forthe fiber dispersion as well as converting a partially frequencymodulated signal into a substantially amplitude modulated signal. Withthe discriminator that counters the dispersion in the fiber, the lasersource may be directly modulated for longer reach applications.

[0012] The discriminator may be a variety of filters such as a coupledmulti cavity (CMC) filter to enhance the fidelity of converting apartially frequency modulated signal into a substantially amplitudemodulated signal as well as introducing enhanced dispersion that isopposite sign of the dispersion in the fiber so that the optical signalmay propagate further distances without being distorted. This inventionmay also provide a modulated laser source that is communicatably coupledto an optical filter where the filter is adapted to lock the wavelengthof a laser source as well as converting the partially frequencymodulated laser signal into a substantially amplitude modulated signal.

[0013] Many modifications, variations, and combinations of the methodsand systems and apparatus of a dispersion compensated optical filter arepossible in light of the embodiments described herein. The descriptionabove and many other features and attendant advantages of the presentinvention will become apparent from a consideration of the followingdetailed description when considered in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

[0014] A detailed description with regard to the embodiments inaccordance with the present invention will be made with reference to theaccompanying drawings.

DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1A illustrate a graph with the transmission and dispersion ofa Fabry-Perot filter with about 1 dB/GHz discriminator slope.

[0016]FIG. 1B illustrates the bandwidth of the Fabry-Perot filter ofFIG. 1A.

[0017]FIG. 2 illustrate a fiber optic system including a directly FMmodulated laser, and a transmission type optical discriminator thatcompensates at least partially for the dispersion of the fiber.

[0018]FIG. 3A illustrates optical signal on a negative transmissionedges of a coupled multi-cavity (CMC) filter versus an opticalfrequency.

[0019]FIG. 3B illustrates corresponding dispersion of the CMC filter inFIG. 3A versus the optical frequency.

[0020]FIG. 3C illustrates optical signal on a positive transmissionedges of the CMC filter according to FIG. 3A.

[0021]FIG. 3D illustrates corresponding dispersion of the CMC filter inFIG. 3A versus the optical frequency.

[0022]FIG. 4 illustrates output waveforms of transmitter, frequencyexcursion and output after filters with positive or negative slope.

[0023]FIG. 5 illustrates a fiber optic system including a directly FMmodulated laser, and a reflection type optical discriminator that alsocompensated partially for the dispersion in the fiber.

[0024]FIG. 6A illustrates optical signal on a negative slope of areflection side of a CMC filter.

[0025]FIG. 6B illustrates the corresponding dispersion of the CMC filterin FIG. 6A versus the optical frequency.

[0026]FIG. 6C illustrates optical signal on a positive slope of areflection side of a CMC filter.

[0027]FIG. 6D illustrates the corresponding dispersion of the CMC filterin FIG. 6C versus the optical frequency.

[0028]FIG. 7A illustrates a structure of a CMC filter.

[0029]FIG. 7B illustrates a structure of a Fabry-Perot filters.

[0030]FIG. 8 illustrates a fiber optic system including a directly FMmodulated laser, and a multicavity etalon filter with dispersion signopposite to that of transmission fiber at a multiplicity of equallyspaced wavelengths.

[0031]FIG. 9 illustrates transmission and dispersion of a multi-cavityetalon filter.

[0032]FIG. 10 illustrates a laser optic system including a circuit forlocking laser wavelength to the edge of a transmission type opticaldiscriminator.

[0033]FIG. 11 illustrates a laser optic system including a circuit forlocking laser wavelength to edge of a reflection type opticaldiscriminator.

[0034]FIG. 12 illustrates a fiber optic system including a directly FMmodulated laser, and a cascade of transmission type opticaldiscriminators having a total dispersion that has opposite sign to thedispersion of the transmission fiber.

[0035]FIG. 13 illustrates a fiber optic system including a directly FMmodulated laser, and a cascade of reflection type optical discriminatorshaving a total dispersion that has opposite sign to the dispersion ofthe transmission fiber.

[0036]FIG. 14 illustrates a fiber optic system including a FM modulatedsource, and an optical discriminator having a total dispersion that hasopposite sign to the dispersion of the transmission fiber.

[0037]FIG. 15 illustrates a fiber optic system including a verticalcavity surface emitting laser source and a method for frequencymodulation.

[0038]FIG. 16 illustrates a fiber optic system including a continuouswave (CW) laser, an external frequency modulator, and an opticaldiscriminator having a total dispersion that has opposite sign to thedispersion of the transmission fiber.

[0039]FIG. 17 illustrates a fiber optic system including a CW laser,semiconductor optical amplifier phase modulator, and an opticaldiscriminator having a total dispersion that has opposite sign to thedispersion of the transmission fiber.

[0040]FIG. 18 illustrates a distributed bragg reflector (DBR) laser andmodulating the gain section for frequency modulation.

[0041]FIG. 19 illustrates a DBR laser and modulating the DBR section forfrequency modulation.

[0042]FIG. 20 illustrates a sampled grating distributed Bragg reflectorlaser (SGDBR) and modulating the gain section for frequency modulation.

[0043]FIG. 21 illustrates a SGDBR and modulating the front DBR sectionfor frequency modulation.

[0044]FIG. 22 illustrates a SGDBR and modulating the rear DBR sectionfor frequency modulation.

DETAILED DESCRIPTION OF THE INVENTION

[0045] This invention provides a laser transmitter system capable ofdirectly modulating a laser source and partially compensating for thedispersion in the fiber so that the system may be applied to faster bitrate and longer reach applications. This may be accomplished byproviding a discriminator that converts frequency modulation (FM) toamplitude modulation (AM) and compensate for the dispersion in theoptical fiber so that the laser source may be directly modulated. Avariety of discriminators may be used such as a coupled multi-cavity(CMC) filter to enhance the fidelity of FM/AM action as well asintroducing enhanced dispersion compensation. By simultaneouslyoptimizing the FM to AM conversion as well as the dispersioncompensation properties, the performance of directly modulating thelaser source may be optimized.

[0046]FIG. 2 illustrates a fiber optic system 100 that includes acurrent modulator 102 that modulates a laser source 104. The currentmodulator 102 may directly modulate the laser source 104. In thisregard, U.S. Pat. No. 6,331,991 by Daniel Mahgereftech, issued Dec. 18,2001 is hereby incorporated by reference into this application. Thelaser source 102 may be a variety of different types of lasers such as asemiconductor laser. The laser may be biased high above the thresholdand the level of modulation may produce a predetermined extinctionratio, such as about 2 dB to about 7 dB. The signal from the laser maythen pass through an optical discriminator 106 with a dispersionD_(discriminator) in ps/nm and the signal from the laser may be passedthrough one of its transmission edges. The optical discriminator 106 mayconvert a partially frequency modulated (FM) signal to a substantiallyamplitude modulated (AM) signal. In this example, the opticaldiscriminator 106 may be a coupled multi-cavity (CMC) filter to enhancethe fidelity of the FM to AM conversion as well as introducing enhanceddispersion compensation to achieve longer reach applications. Theresulting signal from the optical discriminator 106 is transmittedthrough a fiber 108 having net dispersion D_(fiber) in ps/nm. Thediscriminator may have a predetermined dispersion that is opposite signof the dispersion in the fiber, e.g., sign (D_(discriminator))=−sign(D_(fiber)) so that the dispersion effect on the fiber may be minimized.This way, the optical signal may travel further without the signal beingdistorted due to the dispersion in the fiber. The receiver 110 thendetects the signal sent through the fiber 108. When the fiber opticsystem 100 operates in this way, the frequency discriminator 106increases the modulation depth of the incoming laser output in the FM toAM conversion, reduces chirp by rejecting part of the spectrum, as wellas partially compensating for the dispersion in the fiber.

[0047] The discriminator 106 may modify the phase of the incomingelectric field as well as its amplitude. Group velocity dispersion maybe defined as: $\begin{matrix}{{D = {{- \frac{2\pi \quad c}{\lambda^{2}}}\frac{^{2}\varphi}{\omega^{2}}}},} & (1)\end{matrix}$

[0048] where D_(discriminator) is in units of ps/nm that may be positiveor negative depending on the filter shape and frequency as illustratedin FIGS. 3A through 3D. In equation (1): φ is the phase; ω is frequency;c is the speed of light; and λ is wavelength. For D>0, shorterwavelength components of the wave travel faster than the longercomponents, and for D<0, the opposite is true. The discriminator 106 maybe formed by using the transmission edge of a band pass filter. FIG. 3Aillustrates two transmission edges having a positive slope 112 on thelow frequency side, and a negative slope 114 on the high frequency side.FIG. 3B illustrates that the sign of the dispersion D 116 may be afunction of the relative frequency with distinct features having zerosnear the filters transmission edges 118 and 120, respectively on thepositive slope side 112 and the negative slope side 114. The dispersionD 116 is also substantially positive in the pass band on the lowfrequency side 122 and substantially negative on the pass band on thehigh frequency side 124.

[0049]FIG. 4 illustrates the output power 126 and the frequencyexcursion 128 of the laser from the laser source 104 but before thediscriminator 106. After the laser has been passed through thediscriminator 106, the output extinction ratio of the signal 130 isgreater than 10 dB for either a positive slope portion 112 or a negativeslope portion 114 of the discriminator 106. However, the polarity of theoutput depends on the sign of the slope of the discriminator used. For apositive slope portion 112, the polarity is the same as the output fromthe laser source 104, whereas the polarity is opposite for a negativeslope portion 114. As such, the negative slope portion 114 of thediscriminator 106 may be utilized to at least partially compensate forthe dispersion in a fiber having net positive dispersion. As a result ofusing the negative slope portion 114 of the discriminator 106 as afilter, at least some portion of the positive dispersion effect in thefiber may be neutralized so that the signal through the fiber may travellonger distance without becoming distorted. For example, FIGS. 3A and 3Billustrate a spectral position of an optical signal 134 relative to thediscriminator in this configuration. The transmissive portion 136 of theoptical signal 134 experiences a negative dispersion 124, hence loweringthe so-called fiber dispersion penalty and bit error rate ratio at thereceiver. That is, along the optical spectral width over thetransmissive portion 136, the dispersion in the discriminator has anopposite sign compared to the dispersion in the fiber. FIGS. 3C and 3Dillustrate a discriminator response and the spectral position of themodulated laser signal relative to the filter where a non-invertedoutput results from the positive slope portion 112 from thediscriminator 106. The transmissive portion 138 of the signal 140experiences a positive dispersion 123, thereby at least partiallycompensating for fiber having a negative dispersion.

[0050]FIG. 5 illustrates a discriminator 106 that may be used in areflection mode rather than in a transmissive mode as discussed in FIGS.2 and 3. FIGS. 6A and 6B illustrate an optical signal 141 on a negativeslope 142 in a reflective mode of the discriminator. In thisconfiguration, the output 132 from FIG. 4 may be inverted relative tothe input before the discriminator 106. And as illustrated in FIG. 6B,the spectral position of the input signal relative to the discriminatorin a reflective mode may experience a greater negative dispersion thanin the transmission mode. Accordingly, the reflection mode may providefor larger dispersion compensation than in the transmission mode.

[0051]FIGS. 6C and 6D illustrate an optical signal 143 on the positiveslope 144 of the reflection mode of the discriminator. Here, the output130 (FIG. 4) after the discriminator is not inverted relative to theinput. The spectral position of the signal relative to the discriminatoris such that the reflected portion may experience a greater positivedispersion than in the transmission mode. In the reflective mode, thediscriminator may at least partially compensate for the dispersion inthe transmission fiber having net negative dispersion.

[0052] There are a variety of filters that may be used as adiscriminator. For example, the discriminator 106 may be a thin filmdiscriminator that can operate with a FM modulated source at high bitrates with minimal sensitivity to temperature changes. FIG. 7Aillustrates a coupled multi-cavity (CMC) filter 145 that may be used asthe discriminator 106 in the optical system 100. FIG. 7B shows thestructure of a single cavity filter, which forms a Fabry-Perot. The CMCmay be formed by depositing a plurality of thin layers of two materials,such as Ta₂O₅ and SiO₂, having refractive indices, n_(H), and n_(L),where n_(H)>n_(L). When light impinges on such a structure, it partiallyreflects from the interfaces. The interference between these partialreflections produces the desired frequency dependent transmission. TheCMC may be made of a plurality of cavities 147 formed by a spacer layerbetween two multilayer mirrors. Each mirror may be formed by a quarterwave stack (QWS); a stack of alternating layers of high and low indexmaterials, where the optical thickness 149 of the layers may be equal toor about ¼ of the design wavelength in that material. The cavities 147may be either high index or low index material and may be equal to aninterger multiple of ½ wavelength thick.

[0053] A single cavity within the CMC may have the same filter responseas a Fabry-Perot filter 151 as illustrated in FIG. 7B with a large freespectral range on the order of about 100 nm. With multiple cavities inthe CMC, the transmission edges become steeper, while the bandwidthincreases to form a flat-top shape with sharp slopes as illustrated inFIGS. 3A-3D. As a result, the CMC has sharper skirts and wider bandwidthfor high bit rate applications in comparison to a Fabry-Perot device asillustrated in FIG. 1. The number of cavities in the CMC may be adjusteddepending on the application to obtain the desired combination of sharpslope and high dispersion compensation for the signal pass band. Thethickness of the layers, and the material of choice for the cavities maybe also modified to optimize the design. The temperature sensitivity ofthe CMC may be adjusted by the choice of the cavity material andsubstrate. Choosing a material with a low thermal expansion coefficient(TEC) for the cavity produces a CMC with reduced temperaturesensitivity, while choosing a material with high TEC makes the CMC moresensitive to temperature.

[0054]FIG. 8 illustrates a optical system 100 where the discriminator106 may be a multi-cavity etalon (MCE) discriminator that has adispersion D_(discriminator) that is opposite sign to the dispersion ofthe transmission fiber 108 at a multiplicity of equally spacedwavelengths. The MCE discriminator may be applicable in the wavelengthchannels used in telecommunications where a grid is assigned withwavelength separated by 100 GHz. Other wavelength spacings include Δν=25GHz, 50 GHz, and 200 GHz. To decrease the free spectral range of the CMCfor this application, the spacer layers between the mirrors may beincreased to L=c/2n Δν, which corresponds to a length L=1-4 mm forn=1.5. Rather than using a thin film deposition, a stack Fabry-Perotetalons each having thickness on the order of 1-4 mm may be used toprovide the small free-spectral range of about 100 GHz. Increasing thenumber of the etalons in the stack may increase the steepness of thetransmission and the bandwidth may increase slightly, making the MCEdiscriminator applicable to high bit rate applications. As illustratedin FIG. 9, the transmission 148 and dispersion 150 may be periodic. Likethe CMC discriminator, the MCE discriminator may operate in thetransmission edge or reflection edge as discussed in FIGS. 3A to 3D.

[0055]FIG. 10 illustrates a wavelength-locking system 200 where adiscriminator may be used to simultaneously lock the wavelength of thelaser diode. The laser 202 and the discriminator 204 may be mounted onseparate thermoelectric coolers (TECs) 206 and 208, respectively. Aphotodiode 210 may monitor the optical power at the back facet of thelaser 202, and a photodiode 212 may monitor the optical power reflectedfrom the discriminator 204. The wavelength-locking system 200 may alsoinclude a wavelength locking circuit 214 having a comparator 216communicatively coupled to a divider 218 that compares the ratio of thesignals from the two photodiodes 210 and 212. The divider 218 maycompare the ratio of the dispersion in the fiber PD_(filter) 212 to thedispersion in the laser PD_(laser) 210, where the ratior=P_(reflected)/P_(Laser) which may be a substantially fixed set value.The error signal produced in this way may then control the laser TEC 206to adjust the laser temperature and therefore shift the laser wavelengthin order to keep r substantially constant. To avoid wavelength drift,the temperature of the discriminator 204 may be held substantiallyconstant by the thermoelectric cooler 208, and the correspondingtemperature sensors 220.

[0056]FIG. 11 illustrates another wave locking system 230 capable oflocking the laser wavelength to the edge of the discriminator byoperating the photodiode 212 in the transmissive side of thediscriminator 204. As such, the circuit 214 may now measure the portionof the optical power or signal that has been transmitted through thediscriminator 204 using the detector 212 on the transmission side of thediscriminator 204. The divider 218 within the circuit 214 may comparethe ratio of the dispersion in the fiber PD_(transmissive) 212 to thedispersion in the laser PD_(laser) 210, to hold the ratior=P_(transmissve)/P_(Laser) in a substantially fixed set value.

[0057] A variety of optical discriminators with a desired sign ofdispersion may be formed using a variety of filters including a fiberBragg grating filter in transmission or in reflection, a multicavitythin film filter in transmission or in reflection, an arrayed waveguidegrating. A Bragg grating is formed by making a periodic spatialmodulation of the refractive index in a material, such as a fiber or aplanar waveguide. The period of the index may be on the order of λ/2n,where λ is the wavelength of light, and n is the average refractiveindex of the waveguide. FIG. 12 illustrates cascading a plurality ofnon-interfering CMCs, such as a first CMC 300 and a second CMC 302, toobtain a desirable filter characteristic. The transmission function H(Ω)of such cascading filters may be express as a function of frequency Ω,which is the product of the transmission function of the individualfilters. And the dispersion of the cascading filters is the sum of thedispersions of the individual filters. Accordingly, the sum of thedispersions of the cascading filters may be predetermined or designed tohave the opposite sign of the dispersion of the transmission fiber atthe operational wavelength.

[0058] Cascading filters to obtain a desirable dispersion that isopposite of the dispersion in the fiber may offer flexibility indesigning a discriminator with the desirable characteristics. Forexample, filters with sharp slopes may require expanded optical beams sothat the constituent spatial wavelets of the incident beam aresubstantially incident at the same angle. Typical laser beams with afinite spatial profile, such as a guassian, include plane waves having adistribution of wavevectors that have an angular distribution. Thisangular distribution may be determined by the spatial Fourier transformof the beam. With the characteristics of the filter changing slightly asa function of incident angle, the transmission of a beam of finitespatial extent through a filter with sharp spectral features may producea response that may broaden relative to the ideal case. This unwantedbroadening may be voided by producing the desired filter function withsharp slope by a cascading filters with smaller slopes.

[0059]FIG. 13 illustrates a plurality of cascading transmission filters,such as first and second filters 304 and 306, for producing opticaldiscrimination, and a separate reflective type device, such as aGire-Tournois interferometer 106, for dispersion compensation. Thecascading transmission filters may be optimized for their amplituderesponse, and the reflective filter may be optimized for dispersioncompensation. The optical discriminator may be also a multicavity thinfilm filter where change in temperature does not substantially changethe optical spectrum. With the multicavity thin film filter, temperaturestabilization of the filter may not be necessary.

[0060] Optical transmitters may need to operate within a range oftemperatures, such as 0-80° C., to have minimal degradation in theiroutput of optical waveforms. The wavelength of a semiconductordistributed feed-back (DFB) laser may change rapidly with increasingtemperature, typically at a rate of dλ/dT in about 0.1 nm/C. Asdiscussed above FIGS. 3A-3D and 6A and 6D, the point of operation needsto remain substantially fixed as a function of temperature. The point ofoperation is the spectral position of the frequency modulated signal136, 138, 141, or 144 incident on the discriminator relative to the peaktransmission of the discriminator. For example, the optimum point may bethe spectral position of the signal which produces a 3 dB loss afterpassing through the discriminator. The locking circuit illustrated inFIGS. 10 and 11 substantially accomplish this objective with theaddition of circuitry and TEC. In low cost applications, thethermoelectric cooler associated with the DFB laser may be eliminated.In such a case, the multicavity thin film filter or other discriminatormay be predetermined so that it has the same coefficient of thermaldrift dλ/dT as that of the DFB laser. This may eliminate the need forTECs and corresponding control circuits, and keep the laser wavelengthsubstantially fixed relative to the transmission edge of the filter.

[0061] A variety of laser sources may be used with this invention. FIG.14 illustrates a FM modulated source 400 capable of producing an FMmodulated signal. The FM modulated source 400 may be a variety ofdifferent types of lasers, such as: (1) single wavelength semiconductorlasers; (2) external modulation; and (3) tunable semiconductor lasers.There are several types of single wavelength lasers such as DFB edgeemitters and vertical cavity surface emitting lasers (VCSELs). TheVCSELs and DFB may be directly modulated to produce a signal that isfrequency modulated. The VCSELs may be made of two distributed Braggreflector (DBR) mirrors, each formed by a stack of alternating layers ofhigh and low refractive index material to produce high reflectivitymirrors vertical to the growth surface. The gain medium may besandwiched between two such DBR mirrors. FIG. 15 illustrates a combiner504 capable of combining the modulation signal from a driver 500 and dcbias source 502 to provide a summed I_(b)+I_(mod) signal 506 that isused to directly modulate a VCSEL 508. The sum signal or current 506 issupplied to bias the laser above the threshold and modulate its gain,and therefore modulating the frequency of the output to produce apartially frequency modulated signal.

[0062]FIG. 16 illustrates that the optical signal from a continuous wave(CW) source 600 may be externally phase modulated before being filteredby the optical discriminator 602. The output from the CW laser 600 mayenter a phase modulator 604 followed by the optical discriminator 602.An electrical signal from an external driver 606 may drive the modulator604 that may impart a phase shift on the CW signal after the laser. Theoptical discriminator 602 may then convert the FM modulation to AMmodulation and simultaneously provide a partial dispersion compensationfor propagation though the fiber 608 before being detected by thereceiver 610. The optical discriminator 602 may be chosen to have adispersion that is the opposite sign of the transmission fiber 608. Avariety of different types of the external phase modulator 606 may beused, such as a semiconductor modulator, a LiNbO₃ phase modulator, or asemiconductor optical amplifier (SOA). A SOA is normally used to providegain. It is biased at a high current and has substantially more gainthan loss. A care may need to taken to remove feed-back paths to theSOA, or it may become a laser.

[0063]FIG. 17 illustrates that the SOA 700 may be placed after a CWlaser 702 to provide a gain as well as frequency modulation. A combiner704 may combine a modulating current signal I_(mod) from currentmodulator 706 and a bias current I_(b) 708 to provide a summedI_(b)+I_(mod) signal 710. This signal modulates the gain as well as therefractive index of the SOA 702. The index change may produce acorresponding phase change to the incident light and may be used toencode the signal with data. The optical discriminator may convert thephase modulation to amplitude modulation as described above. Thediscriminator may be adapted to have dispersion that is opposite of thedispersion of the fiber at the operational wavelength.

[0064]FIG. 18 illustrates using tunable laser sources for producing theFM modulated signal as well. The laser source producing the FM modulatedsignal may be a distributed Bragg reflector (DBR) laser, where the Bragggrating may be separate from the gain section. By way of background, aDFB laser may be formed from a Bragg grating over the entire laserstructure. A DBR laser generally has three sections: (1) a gain section;(2) a distributed Bragg reflector section; and (3) a phase section.These separate sections may be electrically isolated and biased bydifferent currents. As illustrated in FIG. 18, the current to the gainsection may be modulated to produce an amplitude and frequency modulatedsignal. A modulation signal I_(mod) produced by the driver 800 may becombined with the dc bias current I_(b) from a second source 802 using abias-T or other combiner 804. The sum current I_(b)+I_(mod) 806 may beused to modulate the laser high above threshold as described above for aDFB. The current to the DBR section may be used to tune the centerwavelength, and the phase section may be used to prevent the device frommode hopping, as is discussed in the case of CW.

[0065]FIG. 19 illustrates that the DBR laser may be frequency modulatedby modulating the current of the DBR section that controls thewavelength in the output of the laser. A modulation signal from a driver900 may be combined with a dc current from a source 902 using a combiner904 to drive the DBR section. The dc component may controls the centerwavelength of the operation, and the modulating current may produce thedesired frequency modulation. The output from laser may then passthrough an optical discriminator to produce low-chip pulses with highcontrast ratio.

[0066]FIG. 20 illustrates a laser source that may be a sampled gratingdistributed Bragg reflector laser (SGDBR) 1000. A SGDBR laser 1000 mayhave four sections: (1) A sampled grating in the back; (2) a gainsection; (3) a phase section, and (4) a sampled grating in the front.The function of the gain section and phase section are similar to theDBR laser described above. However, in a SGDBR, the lasing wavelengthmay be determined by both the front and back distributed reflectors. Asampled grating is a grating with a certain periodicity that may haveits index change spatially modulated in order to provide a periodicreflection coefficient.

[0067] The FM modulated signal may be produced in a variety of ways. Forexample, the FM modulated signal may be produced by directly modulatingthe gain section of the laser as in FIG. 20. In such a case, themodulation signal Imod from a driver 1002 may be combined with a dc biasI_(b) from a dc current source 1004 using a combiner 1006, and theresulting sum current I_(b)+I_(mod) may be used to modulate the gainsection. This produces an FM modulated signal that may be inputted tothe optical discriminator as described above.

[0068]FIG. 21 illustrates that the gain section may be biased using a dccurrent source 1200. The front sampled grating section may be suppliedwith a modulated current to produce FM modulated signal. Signal from amodulator 1202 may be combined with a dc current from a dc source 1204using a combiner 1206 and the sum current supplied to the sampledgrating section. The dc bias current may determine the center wavelengthof the output signal together with the current supplied to the backreflector. The modulation signal produces the FM signal needed to besupply the optical discriminator. And as illustrated in FIG. 22, the FMmodulating signal may also be supplied to the back mirror as well.

What is claimed is:
 1. A fiber optic communication system, comprising:an optical signal source adapted to produce a partially frequencymodulated signal; and an optical discriminator adapted to convert thepartially frequency modulated signal into a substantially amplitudemodulated signal, where the optical discriminator is adapted tocompensate for at least a portion of a dispersion in a transmissionfiber.
 2. The system according to claim 1, where the optical signalsource is a directly modulated laser.
 3. The system according to claim1, further including a combiner that combines outputs from a driver anda dc current source, where the driver provides a modulated signal andthe dc current source provides a dc bias current, where the combinercombines the modulated signal and the dc bias signal to provide a summedsignal to directly modulate the optical signal source above itsthreshold and modulate its gain.
 4. The system according to claim 2,where the directly modulated laser is adapted to produce signals with a2-7 dB extinction ratio.
 5. The system according to claim 1, where theoptical discriminator is a thin film filter.
 6. The system according toclaim 5, where the optical discriminator is formed by a transmissionedge of the thin film filter.
 7. The system according to claim 1, wherethe optical discriminator has a positive slope.
 8. The system accordingto claim 1, where the optical discriminator has a negative slope.
 9. Thesystem according to claim 1, where the optical discriminator is formedby cascading a number of non-interfering multicavity thin film filters.10. The system according to claim 1, where the optical discriminator isformed by a coupled multi-cavity filter.
 11. The system according toclaim 1, where the optical discriminator operates in reflection.
 12. Thesystem according to claim 1, where the optical discriminator operates intransmission.
 13. The system according to claim 1, where the opticaldiscriminator is a Bragg grating.
 14. The system according to claim 13,where the Bragg grating is formed in a fiber.
 15. The system accordingto claim 13, where the Bragg grating is formed in a planar waveguide.16. The system according to claim 1, where the optical discriminator isa periodic filter.
 17. The system according to claim 1, where theoptical discriminator is a multi-cavity etalon that has an associateddispersion D_(d) that has the opposite sign to a dispersion D_(f) of thetransmission fiber at a multiplicity of equally spaced wavelengths. 18.The system according to claim 1, where the optical discriminator is aseries of cascaded etalon filters.
 19. The system according to claim 1,where the optical signal source is a single wavelength semiconductorlaser.
 20. The system according to claim 19, where the single wavelengthsemiconductor laser is a distributed feed back laser.
 21. The systemaccording to claim 20, where the single wavelength semiconductor laserincludes a distributed Bragg reflector (DBR) section, a gain section,and a phase section.
 22. The system according to claim 21, furtherincluding a combiner that combines outputs from a driver and a dccurrent source, where the driver provides a modulated signal and the dccurrent source provides a dc bias current, where the combiner combinesthe modulated signal and the dc bias signal to provide a summed signal.23. The system according to claim 22, where the summed signal isprovided to the gain section to produce a partially frequency modulatedsignal above its threshold level.
 24. The system according to claim 22,where the summed signal is provided to the DBR section to produce apartially frequency modulated signal.
 25. The system according to claim20, where the summed signal is provided to the phase section.
 26. Thesystem according to claim 19, where the single wavelength semiconductorlaser is a vertical cavity surface emitting laser.
 27. The systemaccording to claim 1, where the optical signal source is an externallymodulated.
 28. The system according to claim 27, where the opticalsignal source includes a continuous wave laser and a phase modulator.29. The system according to claim 27, where the phase modulator is asemiconductor modulator.
 30. The system according to claim 27, where thephase modulator is a LiNbO₃ modulator.
 31. The system according to claim27, where the phase modulator is a semiconductor optical amplifier. 32.The system according to claim 1, where the optical signal source is atunable semiconductor laser.
 33. The system according to claim 32, wherethe tunable semiconductor laser is a distributed Bragg reflector laser.34. The system according to claim 32, where the tunable semiconductorlaser is a sampled grating distributed bragg reflector (SGDBR) laser.35. The system according to claim 34, where the SGDBR laser includes asampled grating in a rear section, a gain section, a phase section, anda sampled grating in a front section, where a summed signal includes abias current signal and modulated signal that is fed to the gain sectionto produce the partially frequency modulated signal.
 36. A fiber opticcommunication system, comprising: an optical signal source adapated toproduce a partially frequency modulated signal; an optical discriminatorhaving an associated dispersion D_(d) with a either a positive ornegative sign adapted to convert the partially frequency modulatedsignal to a substantially amplitude modulated signal; and a transmissionmedium having an associated dispersion D_(f) with either a positive ornegative sign, where the sign of D_(d) is an opposite sign of D_(f). 37.The fiber optic communication system according to claim 36, where theoptical signal source is a directly modulated laser.
 38. A fiber opticcommunication system according to claim 37, where the directly modulatedlaser is adapted to produce signals with a 2-7 dB extinction ratio. 39.The fiber optic communication system according to claim 36, where theoptical discriminator is at least a portion of a band pass filter. 40.The fiber optic communication system according to claim 39, where theband pass filter operates in reflection.
 41. The fiber opticcommunication system according to claim 39, where the portion of theband pass filter is a high pass filter.
 42. The fiber opticcommunication system according to claim 39, where the portion of theband pass filter is a low pass filter.
 43. The fiber optic communicationsystem according to claim 36, where the optical discriminator is a thinfilm filter.
 44. The fiber optic communication system according to claim43, where the optical discriminator is formed by a transmission edge ofthe thin film filter.
 45. The fiber optic communication system accordingto claim 36, where the optical discriminator has a positive slope. 46.The fiber optic communication system according to claim 36, where thesubstantially amplitude modulated signal has an output extinction ratiogreater than about 10 dB.
 47. The fiber optic communication systemaccording to claim 36, where the optical discriminator has a negativeslope.
 48. The fiber optic communication system according to claim 36,where the optical discriminator is formed by cascading a number ofnon-interfering multicavity thin film filters.
 49. The fiber opticcommunication system according to claim 36, where the opticaldiscriminator is a coupled multi-cavity filter.
 50. The fiber opticcommunication system according to claim 36, where the opticaldiscriminator operates in reflection.
 51. The fiber optic communicationsystem according to claim 36, where the optical discriminator operatesin transmission.
 52. The fiber optic communication system according toclaim 36, where the optical discriminator is a fiber Bragg gratingfilter.
 53. The fiber optic communication system according to claim 52,where the Bragg grating filter is formed in a fiber.
 54. The fiber opticcommunication system according to claim 52, where the Bragg gratingfilter is formed in a planar waveguide.
 55. The fiber opticcommunication system according to claim 36, where the opticaldiscriminator is a periodic filter.
 56. The fiber optic communicationsystem according to claim 36, where the optical discriminator is amulti-cavity etalon where the dispersion D_(d) of the opticaldiscriminator occurs at a multiplicity of equally spaced wavelengths.57. The fiber optic communication system according to claim 55, wherethe optical discriminator is a sampled Bragg grating filter.
 58. Thefiber optic communication system according to claim 57, where thesampled Bragg grating filter is formed in a fiber.
 59. The fiber opticcommunication system according to claim 57, where the sampled Bragggrating filter is formed in a planar waveguide.
 60. The fiber opticcommunication system according to claim 55, where the opticaldiscriminator is a waveguide grating router.
 61. The fiber opticcommunication system according to claim 55, where the opticaldiscriminator is a series of cascaded etalon filters.
 62. The fiberoptic communication system according to claim 36, where the opticalsignal source is a single wavelength semiconductor laser.
 63. The fiberoptic communication system according to claim 36, where the opticalsignal source is a vertical cavity surface emitting laser.
 64. The fiberoptic communication system according to claim 36, where the opticalsignal source is an externally modulated laser.
 65. The fiber opticcommunication system according to claim 64, where the optical signalincludes a continuous wave laser and a phase modulator.
 66. The fiberoptic communication system according to claim 36, where the phasemodulator is a semiconductor modulator.
 67. The fiber opticcommunication system according to claim 36, where the phase modulator isa LiNbO₃ phase modulator.
 68. The fiber optic communication systemaccording to claim 36, where the phase modulator is a semiconductoroptical amplifier.
 69. The fiber optic communication system according toclaim 36, where the optical signal source is a tunable semiconductorlaser.
 70. The fiber optic communication system according to claim 69,where the tunable semiconductor laser is a distributed Bragg reflectorlaser.
 71. The fiber optic communication system according to claim 69,where the tunable semiconductor laser is a sampled grating distributedbragg reflector laser.
 72. A fiber optic communication system,comprising: an optical signal source adapted to produce an optical powerthat is a partially frequency modulated signal; an optical discriminatoradapted to convert the partially frequency modulated signal into asubstantially amplitude modulated signal that splits into a reflectedsignal and a transmissive signal; and a wavelength locking circuitcapable monitoring the optical signal source and the opticaldiscriminator to compare a ratio between the optical power versus one ofthe reflected signal or the transmissive signal to substantiallymaintain the ratio constant.
 73. The system according to claim 72,further including: a first photodiode capable of monitoring the opticalpower from the optical signal source; and a second photodiode on areflected side of the optical discriminator to detect the reflectedsignal, where the wavelength locking circuit is communicatably coupledto the first and second diodes to monitor the optical signal source andthe reflected signal.
 74. The system according to claim 72, furtherincluding a first photodiode capable of monitoring the optical powerfrom the optical signal source; and a second photodiode on atransmissive side of the optical discriminator to detect thetransmissive signal, where the wavelength locking circuit iscommunicatably coupled to the first and second diodes to monitor theoptical signal source and the reflected signal.
 75. The system accordingto claim 72, further including a thermo-electric cooler (TEC) coupled tothe optical discriminator, where the wavelength locking circuit iscommunicateably coupled to the TEC to adjust the temperature of theoptical power to keep the ratio substantially constant.
 76. A fiberoptic communication system, comprising: an optical signal source adaptedto produce a partially frequency modulated signal; and an opticaldiscriminator adapted to convert the partially frequency modulatedsignal into a substantially amplitude modulated signal, where theoptical discriminator is adapted to reflect a portion of the partiallyfrequency modulated signal to produce a reflected signal that is used towavelength lock the partially frequency modulated signal, and where theoptical discriminator is adapted to compensate for at least a portion ofa dispersion in a transmission fiber.
 77. The system according to claim76, further including a wavelength locking circuit adapted to wavelengthlock the partially frequency modulated signal by comparing the partiallyfrequency modulated signal to the reflected signal and then adjustingthe optical signal source to keep the ratio of the partially frequencymodulated signal to the reflected signal substantially constant.
 78. Thesystem according to claim 76, where the optical signal source is coupledto a thermo-electric cooler that adjust the temperature of the opticalsignal source to keep the ratio of the partially frequency modulatedsignal to the reflected signal substantially constant.
 79. A fiber opticcommunication system, comprising: an optical signal source adapted toproduce a partially frequency modulated signal; and an opticaldiscriminator adapted to convert the partially frequency modulatedsignal into a substantially amplitude modulated signal, where theoptical discriminator is adapted to transmit a portion of the partiallyfrequency modulated signal to produce a transmissive signal that is usedto wave length lock the partially frequency modulated signal, and wherethe optical discriminator is adapted to compensate for at least aportion of a dispersion in a transmission fiber.
 80. The systemaccording to claim 79, further including a wavelength locking circuitadapted to wavelength lock the partially frequency modulated signal bycomparing the partially frequency modulated signal to the transmissivesignal and then adjusting the optical signal source to keep the ratio ofthe partially frequency modulated signal to the transmissive signalsubstantially constant.
 81. The system according to claim 79, where theoptical signal source is coupled to a thermo-electric cooler that adjustthe temperature of the optical signal source to keep the ratio of thepartially frequency modulated signal to the transmissive signalsubstantially constant.
 82. A fiber optic communication system,comprising: an optical signal source for producing an optical signal; atransmission medium having an associated dispersion D_(f); a frequencymodulator between the optical signal source and the transmission mediumadapted to at least partially frequency modulated the optical signal;and an optical discriminator having an associated dispersion D_(d)adapted to convert the partially frequency modulated signal into asubstantially amplitude modulated signal, where the associateddispersion D_(d) has either a positive or negative sign, where the signD_(d) is an opposite sign of D_(f).
 83. The system according to claim82, where the optical signal source is a continuous wave source.
 84. Thesystem according to claim 82, where the optical signal source isexternally modulated.
 85. The system according to claim 82, where thefrequency modulator is a semiconductor optical amplifier.
 86. A fiberoptic communication system, comprising: an optical signal source adaptedto produce a partially frequency modulated signal; a first opticaldiscriminator adapted to convert the partially frequency modulatedsignal into a substantially amplitude modulated signal; and a secondoptical discriminator having an associated dispersion D_(d) adapted toreceive the substantially amplitude modulated signal and compensate forat least a portion of a dispersion D_(f) in a transmission medium, whereD_(d) is the opposite sign of D_(f).
 87. The system according to claim86, where the first optical discriminator is a first coupledmulti-cavity (CMC) filter having a first transmission function and afirst dispersion, and the second optical discriminator is a second CMCfilter having a second transmission function, where the first and secondCMC filters have a combined transmission function that is substantiallya product of the first and second transmission functions, and a combineddispersion that is substantially a sum of first dispersion and theassociated dispersion D_(d) of the second optical discriminator.
 88. Thesystem according to claim 86, where the optical signal source is adirectly modulated laser.
 89. The system according to claim 86, wherethe second optical discriminator is adapted to reflect a portion of thesubstantially amplitude modulated signal to produce a reflected signalthat is used to wavelength lock the partially frequency modulatedsignal.
 90. The system according to claim 86, where the second opticaldiscriminator is a Gire-Tournois interferometer.
 91. The systemaccording to claim 86, where the first optical discriminator is adaptedto reflect a portion of the partially frequency modulated signal toproduce a reflected signal that is used to wavelength lock the partiallyfrequency modulated signal.
 92. The system according to claim 86, wherethe first optical discriminator is a multi-cavity etalon filter wherethe dispersion D_(d) of the second optical discriminator occurs at amultiplicity of equally spaced wavelengths.
 93. The system according toclaim 86, where the first optical discriminator is a sampled Bragggrating filter.
 94. The system according to claim 93, where the sampledBragg grating filter is formed in a fiber.
 95. The system according toclaim 93, where the sampled Bragg grating filter is formed in a planarwaveguide.
 96. The system according to claim 86, further including awavelength locking circuit adapted to wavelength lock the partiallyfrequency modulated signal by comparing the partially frequencymodulated signal to a reflected signal and then adjusting the opticalsignal source to keep a ratio of the partially frequency modulatedsignal to the reflected signal substantially constant.
 97. A fiber opticcommunication system, comprising: an optical signal source, where theoptical signal source is adapted to produce a partially frequencymodulated signal; a plurality of cascading transmission filters capableof converting the partially frequency modulated signal to asubstantially amplitude modulated signal; and a reflective filtercapable of compensating for at least a portion of the dispersion in atransmission fiber.
 98. The system according to claim 97, where theplurality of cascading transmission filters are multicavity thin filmfilters that are adapted to maintain their optical spectra substantiallyconstant over temperature changes.
 99. The system according to claim 97,where the reflective filter is a Gire-Tournois interferometer.
 100. Amethod for transmitting optical signal through a transmission fiber,comprising: modulating an optical signal to a partially frequencymodulated signal; converting the partially frequency modulated signal toa substantially amplitude modulated signal; and compensating for atleast a portion of a dispersion in a transmission fiber to transmitfurther the optical signal through the transmission fiber.
 101. Themethod according to claim 100, where the modulating is done directly ata laser source that produces the partially frequency modulated signal.102. The method according to claim 101, where the laser source is asemiconductor laser, and further includes: biasing the semiconductorlaser high above its threshold to produce an extenuation.
 103. Themethod according to claim 100, where the compensating is done byproviding dispersion that is opposite sign of the dispersion in thetransmission fiber.
 104. The method according to claim 100, furtherincluding: reflecting the frequency modulated signal to generate anegative dispersion to compensate for a positive dispersion in thetransmission fiber.
 105. The method according to claim 100, furtherincluding: comparing a ratio between a power of the optical signalversus a reflected portion of the optical signal; and maintaining theratio to substantially wavelength lock the optical signal.
 106. Themethod according to claim 1, where the step of modulating the opticalsignal is done by using a semiconductor laser.
 107. The method accordingto claim 106, further including: comparing a ratio between a power ofthe optical signal versus a transmissive portion of the optical signal;and maintaining the ratio to substantially wavelength lock the opticalsignal.
 108. The method according to claim 107, further including:adjusting the temperature of the semiconductor laser to shift thewavelength of the optical signal to maintain the ratio substantiallyconstant.
 109. The method according to claim 100, where the convertingand compensating is done by a discriminator having a plurality ofinterfering single cavity filters that provide positive and negativetransmission edges and a bandwidth, where each transmission edge has aslope.
 110. The method according to claim 109, where the discriminatoris a coupled multi-cavity (CMC) filter.
 111. The method according toclaim 100, further including cascading a plurality of non-interferingCMC filters to obtain a desirable compensating characteristics.
 112. Amethod for transmitting optical signal through a transmission fiber fora longer reach application, comprising: generating a partially frequencymodulated signal; discriminating the partially frequency modulatedsignal to produce a substantially amplitude modulated signal; andcompensating for at least a portion of a dispersion in a transmissionfiber.
 113. The method according to claim 112, where the generating isdone directly at a laser source that produces the partially frequencymodulated signal.
 114. The method according to claim 112, where thelaser source is a semiconductor laser, and further including: biasingthe semiconductor laser high above its threshold to produce anextenuation.
 115. The method according to claim 112, where thediscriminating compensates for the dispersion in the transmission fiberby providing dispersion in the discriminating that is opposite sign ofthe dispersion in the transmission fiber.
 116. The method according toclaim 112, further including: reflecting the frequency modulated signalto generate a negative dispersion to compensate for a positivedispersion in the transmission fiber.
 117. The method according to claim112, where the discriminating is done by a plurality of interferingsingle cavity filters that provide positive and negative transmissionedges and a bandwidth, where each transmission edge has a slope. 118.The method according to claim 112, where the discriminating is done by acoupled multi-cavity filter.
 119. The method according to claim 112,further including cascading a plurality of non-interfering coupledmulti-cavity filters to obtain a desirable compensating characteristics.120. A method for producing a frequency modulated signal, comprising:alternating high and low refractive index mirrors to produce adistributed bragg reflector (DBR) mirrors; sandwiching a gain mediumbetween two DBR mirrors to provide a laser source; combining a modulatedsignal source and a dc bias source to produce a combined signal; andmodulating the laser source with the combined signal to produce anoptical signal that is biased above its threshold and frequencymodulated.
 121. The method according to claim 120, where the modulatingis done directly at the laser source.
 122. The method according to claim120, where the modulating is done externally from the laser source. 123.A method for producing a frequency modulated signal, comprising:producing a laser; biasing the laser above its threshold level; andmodulating frequency of the laser to produce at least a partiallyfrequency modulated signal.
 124. The method according to claim 123,where the producing is done by a single wavelength semiconductor laser.125. The method according to claim 123, where the producing is done by atunable semiconductor laser.
 126. The method according to claim 123,where the modulating is done directly at the producing laser.
 127. Themethod according to claim 123, where the modulating is done externallyto the producing laser.
 128. The method according to claim 123, furtherincluding: discriminating the partially frequency modulated signal toproduce a substantially amplitude modulated signal; and compensating forat least a portion of a dispersion in a transmission fiber.